Switch for small direct currents



June 26, 1962 Filed Aug. 31, 1960 G. F. PIAZZA SWITCH FOR SMALL DIRECT CURRENTS 2 sheetswsheet 1 I IGLIQEZ INVENTOR. GLan fiance Piazza.

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G F. PIAZZA 3,041,473

SWITCH FOR SMALL DIRECT CURRENTS 2 SheetsP-Sheet 2 G PuLse Gen.

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INVENTOR. Gian Franco P/aizo.

United.

sauna Patented June 25, 1962 hue 3,041,473 SWITCH FOR SR'TALL DIRECT CURRENTS Gian Franco Piazza, Wettingen, Aargau, Switzerland,

assignor to Akticngeseiischaft Brown, Boveri & Cie,

Baden, witzerland, a joint-stock company Filed Aug. 31, 1969, Ser. No. 53,095 Claims priority, applicatinn Switzerland Sept. 1, 1959 5 Claims. ((11. Sill-88.5)

This invention relates to an improved switch of the transistor type for small currents, and also to an arrangement for a plurality of such switches which are actuated in a sequential manner to transmit to a load the respective currents which are applied at the input side of the switches.

An arrangement for transmitting a plurality of measured values in accordance with a time sequence principle is known wherin each measured value is first converted on the transmitter side into a current clearly characterizing its magnitude. These currents are then connected in a sequential manner through use of a selector switch mechanism either directly with a transmission line that leads to a distant receiver, or the currents at the output side of the switch mechanism can first be fed to a device for modulating a carrier wave suitable for transmission over the medium provided. These latter devices can also contain a mutator, or chopper, which serves to transform the variable direct currents into alternating currents or alternating voltages with correspondingly proportional amplitudes.

It is desirable to keep the currents associated with the measured values as small as possible so that they can be derived from the measured values without any great expenditure, and without any adverse effects on the measured values themselves. In transmission arrangements designed for the needs of electric power plant, appear currents having an amplitude preferably up to about ma. The range in which these currents are variable should extend from zero to the respective maximum value, for example 10 ma, so that in the simplest case the currents can be proportional to their respectively associated measured values.

The selector switches used in known arrangements have certain disadvantages. Switches of the mechanical type having mechanically movable contacts are relatively large in size, operate too slowly, have parts which wear out too quickly, and have a tendency to fouling, thus requiring supervision and considerable maintenance. Electronically operated selector switches must be equipped today with transistors. Their temperature characteristic and/or the initial voltage of the emitter-base-gap have therefore prevented the realization of useful switches for processing variable direct currents of the above-mentioned magnitude. Transistor circuits were therefore used only for processing currents above a certain initial value (method of the suppressed zero point).

The above-mentioned disadvantages can be avoided if each measuring circuit is associated with a separate device for modulating a carrier wave, in the simplest case for transformation of the variable direct current into an alternating current and for its amplification. The selector switch would then have to process signals at levels which are by orders of magnitude higher and this could be realized without any difiiculties. As it can be readily seen, however, this measure is unsuitable because of the expenditure which increases with the number of measuring circuits, at least for all those transmission arrangements which transmit not only a very few measuring values.

The switch for small currents in accordance with the present invention is actuated electronically and is equipped with transistors, and opening and closing of the switch to transmit the input current to the output is effected by means of a control current. The improved switch structure makes it possible to build, with silicon transistors that are now available, a selector switching arrangement for more than one hundred measuring circuits without any change of the resistance being noted in the individual measuring circuits during the switching, and with a negligibly low reaction of one measuring circuit on any other measuring circuit. Errors in transmitting current through the selector switching arrangement are below 1% of their maximum values in a temperature range between 20 C. and 55 C. The switching or sequence rate can be increased, if necessary, to more than 10 steps per second. The improved switch thus permits a substantial increase in the transmission rate, and with an accuracy that is sufiicient for all needs. The same accuracy can also be achieved when using the switch in mutators for converting the direct currents to alternating currents.

As will become more apparent from the following desoription of difierent embodiments of the invention, the inventive concept is characterized by a switching device for small currents which is actuated by a control current, and which contains a cascade circuit of two transistors in base connection. The direct current to be controlled by the switch is fed to the input of this cascaded transistor circuit and the output of the latter has connected to it the emitter of a third transistor, the base of this latter transistor being fed periodically with the control current. Also connected to this output is one electrode of a diode, the other electrode of the diode being conneoted to the load. When the control current is absent at the base of the third transistor, the latter is non-conductive and therefore the output from the cascaded transistor circuit passes to the load. Thus, under these conditions, the switch in effect is closed between the input and output sides. When the control current is present at the base of the third transistor, the latter is rendered conductive and the output from the cascaded transistor circuit is thus in efiect shunted away from the load through the third transistor. The switch is thus open. Moreover, according to a feature of the invention, the input to the cascaded transistor circuit is connected over a lrgh resistance to a source of constant current, and similarly the output electrode of the diode is connected over a high resistance to a source or" constant current. The polarities of these two current sources are chosen in such manner that an auxiliary current will be introduced at the input side of the switch and removed at the output side of the switch. The advantage of this arrangement is that the auxiliary current, put through the switch in addition to the measured current to be transmitted therethrough but subtracted again at the output side of the switch before reaching the load, serves to compensate for an inherent non-linear relation which obtains between input current and output current in the starting range of the transistor amplification below about 0.1 ma. Thus, even for low input current values to be transmitted, the switch handles a current value high enough to bring its operation into a linear relation between input and output currents. The switch thus preserves fidelity of transmission of the correct values of the input current to the output load. In order to assure that the auxiliary current tapped from the output side is identical in amplitude with the auxiliary current introduced at the input side, the invention provides, as a further feature, a stabilization of the current sources which is effected by forming them from two parts of a voltage divider resistance, the ends of which are connected to a single current source. Thus any voltage variation in this single source acts uniformly on the introduced and subtracted auxiliary currents.

In the accompanying drawings,

FIG. 1 is an electrical schematic view of one embodiment of the invention;

FIG. 2 is a View showing a modification for that portion of the circuit of FIG. 1 which furnishes the auxiliary currents;

FIG. 3 is an electrical schematic view illustrating the manner in which a plurality of the switches shown in FIGS. 1 or 2 can be arranged for sequential operation as a selector switching device for passing their respectivel applied input currents in sequence to a common transmitter; and

FIG. 4 is also an electrical schematic view ofanothcr embodiment of the invention wherein the improved switch also serves the purpose of transforming the smail order direct current input into an alternating voltage output at an amplitude proportional to the amplitude of the direct current input.

With reference now to FIG. 1, the input current i which is proportional to the magnitude of the measured value desired to be transmitted from one point to another, is connected to a cascaded circuit of two tnansistors T1 and T2. One input terminal is connected to the emitter Tle, the base TM is connected to the emitter T22 and the baseTZb is connected to the other terminal of the input. The two collectors T10 and T20 are connected together at the output point a. V

The current gain of the cascaded circuit of two transistors in base connection is given by ot=oc1+aZ-o4l a2 where or]. and :12 are the current gains of the individual transistors in base connection and have a value of about 0.95 with todays silicon transistors. The value of lies very close to unity, is to a great extent temperaturedependent in the range in question and constant for currents above about 0.1 ma. The direct current input i to be switched, is fed to the input of the cascade circuit, that is, to the emitter of transistor T1. At the output point a of the cascaded circuit is arranged, on the one hand the emitter T3e of another transistor T3, and on the other hand the load A in series with a diode D. The polarities indicated in the drawing apply to use of PNP type transistors. If transistor T3 is in a locked, i.e. a non-conductive state, the output current of the cascaded circuit, which is practically equal to the input current i flows through diode D into the load A. The operating voltage U2 for the cascaded circuit originates from two series-connected voltage portions B1 and B2 of a source indicated by a battery B with a mid tap. If transistor T3 is in an unlocked, i.e. a conductive state, it connects the output of the cascaded transistor circuit directly with a lower potential point U1 represented by the tap point on battery B. The load A and diode D therefore become currentless, and the output current of the cascaded transistor circuit flows oif now over the emitter-collectorgap TSe-TSc of transistor T3. Since the output impedance of the cascaded transistor circuit is very high, there is practically no change in the output current during the switching, notwithstanding the change in potential from U2 to U1. Consequently, no change of the input impedance of the cascaded transistor circuit is noticed during the switching. The state of the control transistor T3, i.e. conductive or non-conductive, is determined by a control current i which is fed to its base T31). in the represented circuit, transistor T3 is non-conductive when the control current i is absent. Thus the switch is, in effect, closed and the current ilows from output point a through diode D and load A. With the control current i present, transistor T3 becomes conductive and the switch is, in effect, open. The current then flows otf from output point a through T3e-T3c to the mid-point on battery B.

As has been previously explained, in the star-ting range of the transistor, current amplification is not constant. Thus, a non-linear relation between the output current and input current obtains in the current region below about 0.1 ma. This disadvantage is eliminated, in accordance with the invention by providing the switch on the input side with an auxiliary current of constant amplitude which is added to the signal input current i having a variable magnitude, but which is again subtracted on the output side so that the current which passes to the load A is still representative in magnitude of the input current. To this end, it will be seen that the input side is connected at the emitter Tle through a high resistance Re with the positive terminal of an auxiliary current source indicated as battery Be, and in a similar manner the output side is connected at the output electrode of diode B through a high resistance Ra with the negative terminal of another auxiliary current source indicated by battery Ba. The negative terminal of battery Be is connected to the positive terminal of battery Ba and this interconnection also leads to the other side of the circuit represented by the negative input terminal and the positive terminal of the voltage supply source B.

' The current sources Be and Ba should be of good constancy so that the additional current tapped from the output side is always equal to the additional current introduced at the input side. If desired, the voltages of the two sources Be and Ba may be stabilized.

In lieu of stabilization of re two sources Be and Ba, FIG. 2 illustrates a modified arrangement wherein a single source for the auxiliary current may be utilized. In this modification the single source, represented by battery Bv, is connected across the end terminals of a voltage divider resistance w, such resistance end terminals also being connected respectively to the high resistance Re at the input and to high resistance Ra at the output. The remainde of the circuit, not shown, is the same as in FIG. 1 and hence has not been illustrated. Thus, any voltage variation in the source Bv will have an equal effect on the introduced as well as on the subtracted currents.

Preferably, the additional currents to be added and subtracted are in the range between about 0.1 ma. and 1.0 ma.

The embodiment illustrated in FIG. 3 is directed to an assembly of a plurality of the switches as shown in FIG. 1 for example, arranged as a selector switching mechanism for a plurality of small direct currents i i and i which are to be connected cyclically and in sequence through switches S1, S2 and S3 respectively to a load such as a transmission line or a modulating device M. Each of these switches S1 to S3 is identical to the switch shown in FIG. 1 or as modified by FIG. 2, and hence in the interest of simplification has been shown in FIG. 3 only in block form. The respective outputs A1, A2 and A3 from these three switches are paralleled and connected to the common load M. The control currents i i and for the switches S1 to S3 respectively, which have the same function as control current i in FIG. 1, are supplied by a generator T having four terminals legended 6, l, 2 and 3. Terminal 0 is common to all the switches, and terminals 1, 2 and 3 are connected respectively to the switches S1, S2 and S3. The arrangement is such that currents 1' i and 1' normally flow and thus serve to keep their respective switches in an open condition. However, generator T includes means for momentarily opening the internal leads to terminals 1, 2 and 3 in a sequential manner so that the switches are closed in the same sequential manner to transmit their respective input currents to the common load M.

FIG. 4 illustrates a modification for the switch illustrated in FIG. 1 which includes a mutator for transforming the small direct current i into an alternating voltage to be fed to the load L, the amplitude of the voltage being proportional to the amplitude ofthe direct current. The switch thus acts as a chopper. To this end, transistor T3 is seen to be locked and unlocked periodically by a sequence of rectangular impulse currents i which are produced in generator G. In the primary winding of a transformer F connected to the output side of the diode D thus flows the chopped input current i The desired proportional alternating voltage appears in the secondary winding of this transformer.

I claim:

1. In an electronic switch for small variable direct currents comprising first and second transistors cascaded in base connection, the emitter of said first transistor serving as one input terminal for the direct current and the base of said second transistor serving as the other input terminal for the direct current, a third switching transistor, the collectors of said first and second transistors being connected to the emitter of said third transistor and to the input electrode of a diode, a load connected to the out put electrode of said diode, means for supplying a control current to the base of said third transistor to render said third transistor alternatively non-conductive or conductive, said third transistor when in a conductive state serving to by-pass the output current from the collectors of said first and second transistors away from said diode and load, and when in a non-conductive state effecting fiow of said output current through said diode and to said load; the improvement which resides in means for introducing an auxiliary direct current of constant amplitude at the emitter of said first transistor and removing said auxiliary current at the output electrode of said diode in advance of said load.

2. An electronic switch for small currents as defined in claim 1 wherein said means for introducing and removing said auxiliary current is comprised of a source of constant current, and circuit means including a high resistance connecting said constant current source to said emitter of said first transistor and to said output electrode of said diode.

3. An electronic switch for small currents as defined in claim 1 wherein said means for introducing and removing said auxiliary current is comprised of a high resistance connected between said emitter of said first transistor and one end of a voltage divider resistor, another high resistance connected between said output electrode of said diode and the other end of said voltage divider resistor, and a source of constant current connected across the end terminals of said voltage divider resistor.

4. A selector switch mechanism comprising a plurality of electronic switches each as defined in claim 1 connected to a common load and which further includes means for applying the respective control currents to said third switching transistor of each of said switches in a sequential manner so as to effect a corresponding sequential operation of said switches.

5. An elecronic switch as defined in claim 1 wherein said load includes a transformer having its primary connected to the output electrode of said diode, and said means for applying a control current to the base of said third transistor is constituted by a source of current pulses whereby the direct current input to said switch is transformed by said load transformer into a proportional alternating current output.

References Cited in the file of this patent UNITED STATES PATENTS 2,817,015 Johnson Dec. 17, 1957 2,829,281 Overbeek Apr. 1, 1958 2,923,819 Schreiber Feb. 2, 1960 

